Please review my fix for 8165372 : "StackWalker performance regression following JDK-8147039"

Bug: https://bugs.openjdk.java.net/browse/JDK-8165372
Webrev: http://cr.openjdk.java.net/~bchristi/8165372/webrev.00/

8147039 reimplemented stack walking using javaVFrames in place of vframeStream, in order to give correct results for the experimental LiveStackFrame feature. However, this also resulted in a significant StackWalker performance regression (25-60%, depending on specific operation and stack depth).

This fix includes stack walking implemented both ways, encapsulated within C++ classes. JavaFrameStream provides the speed we had before for most use cases, while LiveFrameStream provides correct results when using LiveStackFrames.

Performance is much improved, back within 5% or so of pre-8147039 levels, based on my measurements.


1. http://hg.openjdk.java.net/jdk9/hs/hotspot/rev/6174ad93770c

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